feat(37): expose the staged-scene factory reset

App Settings can now restore only the staged canonical scene. The VM saves
pending edits first, resets that scene, reloads immediately, and keeps the same
scene staged. Other scene edits survive; the action is disabled while live or
recording. 615 tests green, clean build with 0 warnings.

Reference: OBS Scene Collections keep scene/source sets distinct:
https://obsproject.com/kb/scene-collections

ViewModels/MainViewModel.cs

@@ -193,6 +193,8 @@ public partial class MainViewModel : ViewModelBase
     public ICommand AddReplayLayerCommand { get; }
     /// <summary>TASK 37: replace the current scene set with its factory snapshot.</summary>
     public ICommand RestoreAllLayoutDefaultsCommand { get; }
+    /// <summary>TASK 37: restore only the staged canonical scene to its factory snapshot.</summary>
+    public ICommand RestoreStagedSceneDefaultsCommand { get; }
     public ICommand ChangeMediaFileCommand { get; }
     /// <summary>Accepts the detector's green/blue-screen offer on a clip (TASK 64).</summary>
     public ICommand AcceptChromaSuggestionCommand { get; }
@@ -275,6 +277,8 @@ public partial class MainViewModel : ViewModelBase
         AddReplayLayerCommand = new RelayCommand(_ => AddReplayLayer());
         RestoreAllLayoutDefaultsCommand = new RelayCommand(_ => RestoreAllLayoutDefaults(),
             _ => IsOffline && !IsRecording);
+        RestoreStagedSceneDefaultsCommand = new RelayCommand(_ => RestoreStagedSceneDefaults(),
+            _ => IsOffline && !IsRecording && StagedScene is { } scene && SceneCatalog.IsCanonical(scene.Name));
         ChangeMediaFileCommand = new RelayCommand(
             element => ChangeMediaFile(element as Source),
             element => element is Source { IsMediaSource: true });
@@ -642,6 +646,16 @@ public partial class MainViewModel : ViewModelBase
         LoadLayout();
     }
 
+    private void RestoreStagedSceneDefaults()
+    {
+        if (!IsOffline || IsRecording || StagedScene is not { } staged || !SceneCatalog.IsCanonical(staged.Name)) return;
+        var sceneName = staged.Name;
+        SaveLayoutNow();
+        if (!_layoutStore.RestoreDefaultScene(staged.Id)) return;
+        LoadLayout();
+        StagedScene = Scenes.FirstOrDefault(scene => SceneCatalog.Is(scene.Name, sceneName)) ?? StagedScene;
+    }
+
     private void ScheduleSave()
     {
         if (_isLoading || _saveDebounce == null) return;